In a typical imaging application, a solid state image sensing device, in the form of an integrated circuit die (chip), is attached to a cavity in a chip carrier package. The position of the image sensing device is located approximately within an alignment pattern (typically, a cross hair pattern) that is etched in the bottom of the cavity of the chip carrier package. In a typical assembly operation, a video camera is used to find a cut edge of the die as well as the chip carrier alignment pattern. The image sensing device is then centered relative to the alignment pattern and potted in place in the carrier package with glue. The package is usually a laminated ceramic (cerdip), a metal lead frame with ceramic or plastic attached on the top and bottom of the lead frame (plastic dip), or a plastic dual-in-line package. The image sensing device is finally sealed in the chip carrier cavity with a cover glass to protect the image sensing device from contamination.
This entire image sensor (die/carrier) assembly is approximately located in the optical system that is to use it and the image sensing device is then energized. The optical system images one or more imaging targets upon the image sensor assembly, which produces corresponding image signals that are used to precisely position the image sensor assembly in the optimum focus position for the optical system. It is customary for this positioning process to exactly locate the image sensor assembly within the optical system relative to the usual six degrees of positional freedom. This active alignment technique, however, requires extensive fixturing and a significant time to complete. The carrier package may have significant variation, up to .+-.10%, due to the process used to fabricate the carrier packages. In particular, there may be significant variation between the cut edges used in alignment and the optically active pixel sites in the image sensing device. Moreover, the image sensor assembly process restricts subsequent alignment inspection due to the alignment marks being obscured under the glue used to bond the image sensing device to the carrier package. Inspection with a video camera after assembly for device placement accuracy is therefore difficult or impossible.
Japanese patent publication 2112280 (1990) suggests use of exact constraints to mechanically locate the cut edges of the image sensing device to features machined in a metal plate that composes the base of the carrier package. An additional locating feature is cut into the edge of the machined metal plate for use as a positioning reference with respect to the optical system in the final product. The patent publication discloses use of the cut edges to locate the image sensing device; however, the aforementioned problem remains, that is, the possibility of significant variation between the cut edges used in alignment and the optically active pixel sites in the image sensing device. Moreover, the locating feature cut into the edge of the machined metal plate neither increases the existing precision nor does it inherently provide exact constraint of the carrier package in the final product.
To simplify production assembly and to allow for image sensor (die/carrier) assembly replacement in the field, it would be desirable to use a passive alignment technique, not requiring video imaging, in which external reference features/datums on the sensor package are used throughout the process, that is, in the sensor assembly process as well as in the final product assembly process. Ideally, the chip carrier package design would specify a standard set of reference features/datums for fabricating and assembling the sensor package, and for positioning the package in the final product.